The present invention relates to a display device and a method for fabricating the same, more specifically a self-emission type display device using an electroluminescence (EL) element, etc., and a method for fabricating the same.
An active matrix type display device includes a plurality of switching elements and pixels respectively operated by the switching elements, which are formed in a matrix on a transparent insulating substrate.
The typical active-matrix type display device is represented by liquid crystal display device. The liquid crystal display device includes a liquid crystal filled between one substrate having the switching elements and another substrate opposed thereto and controls the liquid crystal by the switching elements to use the liquid crystal as shutters.
As display devices other than the liquid crystal display device are known self-emission type display devices, such as EL (electroluminescence) displays, FED (field emission displays) etc. In the EL display, light emitting elements of an inorganic or an organic material connected to the switching elements are caused to emit the light, no liquid crystal material is required to be filled, and no substrate to be opposed to the light emitting elements is required. The FED uses light generated when electrons from electron emitting devices are caused to impinge upon a fluorescent object provided on an opposed substrate.
Among such active matrix type display devices is recently noted a display device using organic electroluminescence elements (organic EL elements) as active driven elements, and polycrystalline silicon TFTs as the switching elements. The EL element, which is different from liquid crystal devices using as shutters the liquid crystal molecules sandwiched between two substrates, self-emits the light, and has the advantages that the EL does not have visual field angle dependency, and can be completed with one substrate and can be accordingly thinner and lighter than the liquid crystal display device, and other advantages. In the active drive, the EL element can be caused to emit the light continuously during 1 frame, and the emission period of time can be made long. Thus, emission qualities, such as luminance, etc., can be high, and the emission period of time is long, whereby the current which instantaneously flows can be small, and the lifetime can be long.
A display device using the organic EL element will be explained below with reference to FIGS. 13 and 14. The display devices using the organic EL elements are classified depending on directions of emission of light in bottom emission type, which emits light toward the side of the substrate with the switching elements formed on and in top emission type, which emits light toward the side opposite to the substrate with the switching elements formed on. The bottom emission type-display devices are described in, e.g., Reference 1 (T. Shimodaet al., “High resolution light emitting polymer display driven by low temperature polysilicon thin film transistor with integrated driver”, ASIA DISPLAY 98, pp. 217-220) and Reference 2 (M. T. Johnson et al., “Active matrix poly LED display”, IDW′00, pp. 235-238). The top emission-type display devices are described in, e.g., Reference 3 (Japanese published unexamined patent application No. 2001-0600076), Reference 4 (Japanese published unexamined patent application No. 2001-147659), and Reference 5 (T. Sasaoka et al., “A 13.0-inch AM-OLED display with top emitting structure and adaptive current mode programmed pixel circuit (TAC)”, SID 2001, pp. 384-386).
FIG. 13 shows the conventional bottom emission type display device.
A channel layer 102 of polycrystalline silicon film is formed on a transparent substrate 100. A gate electrode 106 is formed on the channel layer 102 with a gate insulating film 104 interposed therebetween. A source region 108 and a drain region 110 are formed in the channel layer 102 on both sides of the gate electrode 106. Thus, a thin-film transistor (TFT) having the channel of polycrystalline silicon is constituted. An inter-layer insulating film 112 is formed on the transparent substrate 100 with the thin film transistor formed on. Contact holes 114, 116 are formed in the inter-layer insulating film 112 down to the source region 108 and the drain region 110. On the inter-layer insulating film 112, a source electrode 118 and a drain electrode 120 are formed, respectively connected to the source region 108 and the drain region 110 through the contact holes 114, 116.
An inter-layer insulating film 122 is formed on the inter-layer insulating film 112 with the source electrode 118 and the drain electrode 120 formed on. A contact hole 124 is formed in the inter-layer insulating film 122 down to the drain electrode 120.
An anode electrode 126 electrically connected to the drain electrode 120 through the contact hole 124, a hole transporting layer 128, a light emitting layer 130, an electron transporting layer 132 and a cathode electrode 134 are sequentially laid on the inter-layer insulating film 122. Thus, an organic EL element connected to the TFT is constituted. In the bottom emission type display device, in which light is emitted downward of the transparent substrate 100, the anode electrode 126 is formed of a transparent electrode material, e.g., ITO or others, and the cathode electrode 134 is formed of a high reflectance material, e.g., aluminum.
A protection layer 136 is formed on the transparent substrate 100 with the TFT and the organic EL element formed on. Usually, a glass substrate is used as the protection layer 136. A sealing compound 138 is provided, surrounding the device forming region between the inter-layer insulating film 122 and the protection layer 136, so as to protect the organic EL element from the outside water, etc.
FIG. 14 shows the top-emission type display device.
As shown in FIG. 14, the top emission type display device is the same as the bottom emission type display device in the basic structure. The top emission type display device is different from the bottom emission type display device in that the layer structure of the organic EL element thereof is opposite to that of the bottom emission type display device.
That is, in the top emission type display device, a cathode 134 electrically connected to the drain electrode 120 through the contact hole 124, the electron transporting layer 132, the light emitting layer 130, the hole transporting layer 128 and the anode electrode 126 are sequentially formed on the inter-layer insulating film 122. In the top emission type display device, in which light is emitted upward of the transparent substrate 100, a transparent electrode material, e.g., ITO or others is used as the anode electrode 126, and a high reflectance material, e.g., aluminum or others is used as the cathode electrode 134.
Pixels 140 each comprising one TFT and one organic EL element as described above are arranged in a matrix on the transparent substrate 100 as shown in FIG. 15. On the transparent substrate 100 there are formed a plurality of data bus lines 142 commonly connected the source electrodes 118 of the TFTs of the pixels adjacent to each other in the direction of the columns as viewed in the drawing, and a plurality of scan bus lines 144 commonly connecting the gate electrodes 106 of the TFTs of the pixels adjacent to each other in the direction of the rows as viewed in the drawing. At the periphery of the pixel region, a data control circuit 146 connected to the data bus lines 142 and a gate control circuit 148 connected to the scan bus lines 144. The data control circuit 146 and the gate control circuit 148 are connected to a flexible printed circuit board 150 which connects the former to an outside circuit board including a drive circuit, etc.
The above-described display device is used widely as monitors such as 1) stand alone monitors and monitors built in devices, walls, etc., 2) monitors having the displays folded inside as in notebook type personal computers, etc., 3) monitors of mobile devices, such as portable telephones, etc., and 4) tablet PCs and liquid crystal TVs. The unit forming the devices described above in 1) to 4) is called a module: a) a display substrate with the EL elements mounted on, b) a printed circuit board (outside circuit substrate) with control circuits, etc. mounted, which is connected to a display substrate with a freedom degree of bending, etc., and others which are accommodated compact in an outer frame called a bezel.
As methods for connecting the display substrate and the outside circuit substrate are 1) the method of connecting the circuit substrate mounted on a printed circuit board, etc. and the display substrate by the flexible printed circuit board with an anisotropic conducting resin, solder or others, 2) TAB (Tape Automated Bonding) and COF (Chip On Film), which mount parts of circuits mounted on a flexible printed circuit board or a film substrate to connect the same to the periphery of the display substrate, 3) COG (Chip On Glass), which mounts semiconductor devices directly on the display substrate, etc. The printed circuit boards with circuits mounted, which are common with these methods, are connected to the display substrates by the flexible films with freedom degrees of bending or others. In COG, the semiconductor elements are mounted on the display substrate in a region other than the pixel region.